1. Field of the Invention
The present invention broadly relates to the art of magnetic separation, and more specifically to a system and method that utilize magnetic separation techniques to separate a given feedstock into its magnetic and nonmagnetic components.
2. Relevant Art
Magnetic separation technology exploits the difference in magnetic properties between the magnetic and nonmagnetic components of a given feedstock.
Magnetic susceptibility is a general reference given to a particle's magnetic or nonmagnetic qualities. In the magnetic separation arena, a given feedstock is defined by its degree of magnetic susceptibility. This degree of magnetic susceptibility is an important factor in magnetically separating various components of the feedstock. Generally, lower strength magnets are employed early in the magnetic separation process to separate highly magnetic fractions from the feedstock. One or more additional stages of separation are then employed using stronger magnetic fields for separating less magnetically susceptible particles.
Another physical property affecting magnetic separation is the size, mass, or both size and mass of particles. Today, in general, the average particle size of processed minerals is finer than that processed in the past. As particle size decreases, conventional methods of dry magnetic separation become less effective. Three types of magnetic separator used for dry magnetic separation are the rare earth roll (“RER”), the recently developed HE10, and the rare earth drum (“RED”).
Multi-Stage Processing
In a magnetic separation process, general practice is to utilize multi-pass or multi-stage processing. Multi-stage processing involves passing feedstock through a first stage of processing to split the feedstock into two or more streams. Typically these streams are broadly termed magnetic and non-magnetic. In this single split, two-product example, the resulting streams are then passed through another stage of magnetic processing to separate any magnetic minerals left in the nonmagnetic stream and to separate any nonmagnetic minerals left in the magnetic stream.
Multi-stage processing usually improves overall grade and recovery aspects of a magnetic separation process. In many cases, two or more stages of magnetic separation take place within the cabinet of a single industrial machine. This is known as a “non-magnetic retreat” configuration, which means that the non-magnetic product from a first stage of magnetic separation is re-treated in a second stage, and the non-magnetic product from the second stage is re-treated in a third stage. Note that, based on the exact mineral composition at a specific stage of processing, a different magnet design can be selected for that stage to optimize the separation efficiency of the overall multi-stage process.
The Conventional Rare Earth Roll
A conventional RER has a cassette assembly that supports an idler shaft and a hinged support mechanism. The shaft is connected to a cylindrical magnetic roll that rotates on its longitudinal axis. A belt, monitored by a tracking system, is installed over the magnetic roll, the idler roll, and cassette. The magnetic roll rotates, which in turn drives the belt, and the belt in turn drives the idler roll. Most often, the magnetic section of the RER is made from a combination of high strength permanent magnet rings and steel pole rings arranged to maximize magnetic force on the outer surface of the belt.
Generally, a vibratory or rotary feeder and a feed chute are used to present a continuous stream of feedstock directly onto the surface of the belt of the RER, near the idler roll. The feed is presented to the belt in the same direction as the motion of the belt. The velocity of the feedstock is closely matched to the velocity of the belt surface to minimize both the wearing of the belt and the skipping or bouncing of the particles. From the idler roll, the belt transports the material to the magnetic roll. The travel time allows the feedstock to settle, thus maximizing separation efficiency. As the feedstock travels over the magnetic roll, the magnetic particles are attracted to the magnet and tend to stick to the surface of the belt when atop the roll. Non-magnetic particles are carried away from the roll by their own momentum. Particles of the feedstock take different trajectories based on their degree of magnetic susceptibility and other physical properties, such as mass, shape, and density. One or more adjustable splitters are positioned below the magnetic roll to collect the particles in different hoppers. The most common arrangements are to have either one or two splitters that divide the material into either two products of magnetic and non-magnetic, or into three products of magnetic, non-magnetic, and middlings.
The RER has the advantage of high magnetic strength given that the inner surface of the belt is in direct contact with the outer surface of the RER magnet. In addition, the belt can be very thin, which provides for little or negligible interference with magnetic forces. Unfortunately, the resulting strong magnetic forces also attract fine magnetic dust to the magnetic roll, which then collects on the underside of the RER's belt as well. This decreases belt life and reduces separation efficiency. When the belt is replaced, the magnetic roll can be manually cleaned to rid it of the accumulation.
Also detrimental to separation efficiency in an RER is static charge buildup between the outer surface of the belt and fine particles. While feedstock travels the length of the belt to the magnetic roll, particles rub together creating a static charge that causes the fine non-magnetic, non-conductive particles to stick to the belt surface, thereby contaminating the magnetic stream and inhibiting proper separation.
Another disadvantage of the RER arises from the need to regularly replace the belt. Changing the belt on an RER can be tricky. If not done properly, the belt can develop folds, wrinkles, or tears leading to the belt's early failure. In addition, the idler roll and belt tracking system contain many additional parts that need to be monitored and maintained. Proper maintenance is important as the belts, and the mechanisms in place for their use, are costly.
The HE10
An HE10 is a variation of a conventional RER that, in general, uses an innovative method of supplying feedstock to an RER to increase separation efficiency. (For details, refer to U.S. Pat. No. 7,296,687 to Arvidson et al.) The HE10 accomplishes this by positioning a continuous stream of feedstock onto the belt of the HE10 at points where the belt crosses the magnetic roll. The feedstock is directed to selectable positions on the belt, at selectable angles of impact. Enhanced separation of the particles of the feedstock results from the combined forces of the feedstock impacting the belt, the resulting bounce of the feedstock from the belt, the force of gravity, and the simultaneous magnetic attraction of the magnetic roll.
The HE10 provides a strong magnetic force that permits the processing of fine particles of feedstock while also maintaining a reduced static buildup among the particles. The HE10 also contains improved dust control elements that help stave off the accumulation of particles on the underside of the belt and on the magnetic roll that can lead to premature belt wear and a loss of separation efficiency. The HE10 does not, however, completely prevent the accumulation of particles on the underside of the belt.
The Conventional Rare Earth Drum
A conventional RED has a shell that is thin, non-magnetic, and highly resistant to wear. The cylindrical shell is rotated longitudinally on a shaft via end plates and bearings using a drive system commonly consisting of a motor and a gearbox sometimes aided by drive belts and pulleys. The shaft remains stationary and supports a magnet assembly within the shell. The magnet assembly usually has a pie shape when viewed from its end, with the radius of the magnet assembly closely matching the inside radius of the shell. To maximize magnetic effect, the clearance between the magnet and shell is adjustable. Clearance between the inside of the shell and the surface of the magnet is generally minimized so that the strength of the magnetic field outside of the shell is maximized. Most often, the magnet assembly is made up of a combination of high strength permanent magnet blocks arranged in such a way as to maximize the strength of the magnetic field outside of the shell. All parts together are called a drum.
Generally, a vibratory or rotary feeder and a feed chute are used to present a continuous stream of feedstock directly onto the surface of the rotating shell of the RED, generally at a twelve o'clock position. The feedstock is presented to the drum in a direction that is approximately tangent to the shell surface, in the direction of rotation. The velocity of the feedstock is closely matched to the velocity of the drum to minimize both the wearing of the shell surface and the skipping or bouncing of the particles. As the material travels on the surface of the shell, the magnetic particles are attracted to the magnet assembly within the drum and so stick to the shell. Non-magnetic particles are carried away from the drum by centrifugal force. Particles of the feedstock take different trajectories based on their degree of magnetic susceptibility and other physical properties, such as mass, shape, and density. One or more adjustable splitters are positioned below the magnetic drum to collect the particles in different hoppers. The most common arrangements are to have either one or two splitters that divide the material into either two products of magnetic and non-magnetic, or into three products of magnetic, non-magnetic, and middlings.
The RED is most often used on feeds of larger sized particles. REDs with low intensity magnetic fields are used to sort highly magnetic material from feedstock. These REDs also are often used to protect feedstock from “tramp iron.” Examples of tramp iron are pieces of machinery, nuts, bolts, and similar items that should be removed from the feedstock to ensure safety and quality of separation. Other REDs with higher intensity magnetic fields are used to concentrate various types of magnetic minerals and to separate less magnetic materials.
An RED's shell is thick enough to endure significant forces and wear. The shell is also impervious to the buildup of static charge. In addition, the drum of an RED is closed, keeping dust from collecting on the magnet assembly inside. An inherent disadvantage of the RED, however, is that its thicker shell reduces the strength of its magnetic field at the outside surface of the shell. Adding to this disadvantage is the requisite clearance between the magnet assembly and the inner surface of the shell.
What is needed is a magnetic separation system that reduces or eliminates static buildup and belt wear issues that can lead to increased operating costs and downtime, while also providing the magnetic field strength necessary to effectively separate both large and fine particles of a given feedstock.